jpa entity cache serialization how it works

i'm studying entity cache.

(kotlin, spring, jpa, hazelcast as second cache <= environment)

in data class, if i cache this entity, it makes this error(class com.exercise.Test cannot be cast to class java.io.Serializable)

@Cacheable
@Cache(
    usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE,
    region = "Test"
)
@Entity
@Table(name = "the_test")
data class Test(

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Int = 0,

    val locale: Locale,

    val testJson: TestJson
)

enum class Locale {

    JP,
    KO,
    EN,
    ;
}

typealias TestJson = TestData

data class TestData(
    val name: String = "",
    val type: String = "TestData",
    val children: List<TTestData> = emptyList()
)

data class TTestData(
    val name: String,
    val type: String = "TTestData",
    val isShow: Boolean,
    val payload: Optional<Any>,
    val children: Optional<List<TestData>>
)

but after i added to each nested TestJson like below, then it works.

typealias TestJson = TestData

data class TestData(
    val name: String = "",
    val type: String = "TestData",
    val children: List<TTestData> = emptyList()
):Serializable

data class TTestData(
    val name: String,
    val type: String = "TTestData",
    val canBeRecentMenu: Boolean,
    val payload: Optional<Any>,
    val children: Optional<List<TestData>>
):Serializable

i still don't get it why it works fine and why serializable needed like this.

can anyone explain this?

1. because of hazelcast built-in serialization, entity didn't need implements serializable?
2. enum class didn't need implements because of kotlin serialization?
3. why each testjson need serialization? how do i know that it needs implements serializable? i thought nested class is also class so hazelcast built-in serialization works too, but it didn't. it makes me confuse